Separate drupal core project logic

Problem/Motivation

Drupal core is a unique project. When testing just core, you would enter "drupal" from the project. This throws an error because it basically tries to require drupal inside of the existing drupal project :)

Error: "Root package drupal/drupal cannot require itself in its composer.json"

Furthermore, if drupal core is the project, the core selector under the advanced options is therefore extraneous.

Steps to reproduce

Enter drupal as the project, submit

Proposed resolution

1. Add specific logic for drupal core that would vary from other projects and/or distributions

2. Hide the core selector if/only if Drupal is selected as the project, show it if the project changes to something else besides drupal

Remaining tasks

1. React changes for conditional

2. Backend/tugboat changes for conditional logic of drupal itself

3. Automated tests

User interface changes

Conditionally showing the core selection based on project selected

API changes

n/a

Data model changes

n/a